This invention relates to a process for producing a colored toner to be used in electrophotography, electrostatic recording, electrostatic printing or toner-jet recording printing operations. More particularly, the invention provides a melt-dispersion process for producing a colored toner wherein the toner comprises particles of a controlled particle size and a narrow particle size distribution.
Electrophotography is a process that employs a photoconductive material to form an image by forming an electrostatic latent image on a photosensitive member by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, transferring the toner image to a recording medium such as paper as the occasion arises, and thereafter fixing the toner image by the action of heat, pressure or solvent vapor. As methods for developing the electrostatic image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed, and methods suited for the respective image forming processes are employed. In recent years, higher-speed copying, higher image quality and color image formation are required for electrophotography. This has placed a demand in the art for developing colored toners that can perform satisfactorily in such processes.
The toner used in electrophotography applications typically comprises a colorant (which includes the color black) dispersed in a resin having a binding property referred to as a binder resin and, optionally, contains various additives such as a charge controller. In use, the toner is typically charged by triboelectric charging and supplied while being carried on a developing roller or the like to the surface of the photoreceptor.
One method of producing toners is by melt-kneading colorants such as dyes and pigments into thermoplastic resins to effect uniform dispersion, followed by pulverization and classification using a fine grinding mill and a classifier, respectively, to produce toners having the desired particle diameters. According to the pulverization process, an admixture of toner raw materials such as binder resin and a colorant is melt-kneaded, and a melt-kneaded product thus obtained is cooled down to be solidified, followed by pulverization and classification, in a consequence whereof a toner is obtained. The diameter-reduced toner manufactured by the pulverization method contains particles of which shapes are not uniform, and has extremely deteriorated powder flowability. When such a toner is used, the toner is unevenly charged before supplied to an electrostatic latent image, for example, which possibly generates unevenness in density or color of an image being formed.
Other drawbacks of the pulverization process include a limit to the range in which toner materials are selected. For example, colorant-dispersed resin materials must be brittle enough to be pulverizable with ease by means of an economically usable production apparatus. Since the colorant-dispersed resin materials must be made brittle to meet such a requirement, a group of particles having a broad particle size distribution tends to be formed when such a resin material is actually pulverized at a high speed, especially causing a problem that extremely fine particles having been pulverized excessively are included in this group of particles in a relatively large proportion. Such highly brittle materials tend to be further finely pulverized or powdered when used actually for the development in copying machines or the like.
In addition, the resolution, solid-area uniformity and gradation reproducibility of images formed by toners commonly depends on the properties of toners, especially their particle diameter, in a large proportion, where the use of toners with a smaller particle diameter brings about images with higher quality. Accordingly, recently developed printers and high-grade copying machines perform better with toners having a small particle diameter; however, in making toner particles having a smaller particle diameter by the pulverization process, the cost of pulverization would increase exponentially while the classification yield would be reduced precipitously, which makes the conventional production method cost prohibitive.
To overcome problems associated with the pulverization process, various toner production processes have evolved through the years. One method is a suspension polymerization method wherein a monomer mixture, most commonly a mixture of styrene monomer, acrylic monomers, a polymerization initiator, a colorant, and other ingredients are evenly mixed into an organic liquid phase, followed by dispersing the organic liquid phase in aqueous media and polymerizing the organic liquid phase. A significant drawback of this method is that the binder resin is limited to vinylic polymers such as polystyrene-acrylate copolymers, which can be manufactured by radical polymerization. The toner containing the vinylic polymer as binder resin is limited in printing performances such as high speed fusing properties and color chromaticity.
Another method is emulsion agglomeration method wherein a polymer colloid and a pigment colloid are mixed together, then induce the mixture to go through a controlled coagulation to form agglomerated particles to obtain a toner. The drawback of this method is that the binder colloid is limited to polystyrene-acrylate copolymers, and the composition of each toner particle tends to have large variation.
Yet another method is solvent dispersion method wherein a binder resin is dissolved in a water immiscible organic solvent solution containing dispersed colorant and other ingredients, then disperse the organic solution in water with the aid of a dispersant, followed by removal of the organic solvent to obtain a dispersion of toner particles in water, followed by washing and drying. The method allows the use of a broader choice of binder resins including the more desirable polyester resin. However, a significant drawback of this method is the employment of a substantial amount of organic solvent which is environmentally undesirable. Furthermore, it is difficult to completely remove the solvent from the toner particles.
A method that avoids much of the above drawbacks is the melt dispersion process in which a molten mixture comprising binder resin and colorant is dispersed under agitation in water in the presence of a dispersant. The types of dispersants employed in the prior art vary. For example, U.S. Pat. No. 3,669,922 discloses a process that comprises a controlled heating, melting, and dispersion of a polymer in the presence of water and a nonionic surfactant; U.S. Pat. No. 3,422,049 discloses a process that comprises a controlled heating, melting, and dispersion of a polymer in the presence of water and a block copolymer of ethylene oxide and propylene oxide; U.S. Pat. No. 4,440,908 discloses a process that comprises a controlled heating, melting, and dispersion of a polymer in the presence of water and a dispersing amount of a substantially water insoluble ionomer polymer such as, for example, polyethylene; U.S. Pat. No. 4,610,944 discloses a process that comprises a controlled heating, melting, and dispersion of a polymer in the presence of water and fine powdery inorganic particles; and U.S. patent application Publication No. 2007/0202433 discloses a process of manufacturing a toner comprising granulating by applying a shearing force and a collision force to a kneaded product of toner raw material containing binder resin and a colorant in water under heat and pressure and in the presence of a water-soluble polymer dispersant such as, for example, one or more water-soluble polymeric dispersants selected from polyoxyalkylene alkylarylether sulfate salt and polyoxyalkylene alkylether sulfate salt. In such prior art methods, however, it is difficult to achieve particle sizes below 12 μm with a narrow size distribution, which are needed to achieve the printing definition and resolution demanded by the current state of the art of printers and copiers. Moreover, the process described in U.S. patent application Publication No. 2007/0202433 employs a large amount of water-soluble polymeric dispersant which is often difficult to remove in the final product even by a subsequent washing process, thus, charge characteristics of the toner particles vary widely with the residual level and the type of the dispersants used. Accordingly there is a need in the art for a process for producing toner that does not suffer from the drawbacks associated with the prior art processes.